Such ion traps may be used in order to provide a buffer for an incoming stream of ions and to prepare a packet with spatial, angular and temporal characteristics adequate for the specific mass analyser. Examples of mass analysers include single- or multiple-reflection time-of-flight (TOF), Fourier transform ion cyclotron resonance (FT ICR), electrostatic traps (e.g. of the Orbitrap type), or a further ion trap.
A block diagram of a typical mass spectrometer with an ion trap is shown in FIG. 1. The mass spectrometer comprises an ion source that generates and supplies ions to be analysed to a single ion trap where the ions are collected until a desired quantity are available for subsequent analysis. A first detector may be located adjacent to the ion trap so that mass spectra may be taken, under the direction of the controller. The mass spectrometer as a whole is also operated under the direction of the controller. The mass spectrometer is generally located within a vacuum chamber provided with one or more pumps to evacuate its interior.
Ion storage devices that use RF fields for transporting or storing ions have become standard in mass spectrometers, such as the one shown in FIG. 1. FIG. 2a shows a typical arrangement of four electrodes in a linear ion trap device that traps ions using a combination of DC, RF and AC fields. The elongate electrodes extend along a z axis, the electrodes being paired in the x and y axes. As can be seen from FIG. 2a, each of the four elongate electrodes is split into three along the z axis.
FIGS. 2b and 2c show typical potentials applied to the electrodes. Trapping within the storage device is achieved using a combination of DC and RF fields. The electrodes are shaped to approximate hyperbolic equipotentials and they create a quadrupolar RF field that assists in containing ions entering or created in the trapping device. FIG. 2c shows that like RF potentials are applied to opposed electrodes such that the x axis electrodes have a potential of opposite polarity to that of the y axis electrodes. This trapping is assisted by applying elevated DC potentials to the short end sections of each split electrode relative to the longer center section. This superimposes a potential well on the RF field.
AC potentials may also be applied to the electrodes to create an AC field component that assists in ion selection.
Once trapped, ions may be later ejected to a mass analyser either axially from an end of the ion trap or orthogonally through an aperture provided centrally in one of the electrodes.
This type of ion trap is described in further detail in U.S. Pat. No. 5,420,425.
The ion trap may be filled with a gas such that trapping of ions is assisted by the ions losing their initial kinetic energy in low-energy collisions with the gas. After losing sufficient energy, ions are trapped within the potential well formed within the ion trap. Those ions not trapped during the first pass are normally lost to the adjacent ion optics.
For most ions, over a wide range of masses and structures, substantial loss of kinetic energy occurs when the product of gas pressure and distance traveled by the ions (P×D) exceeds around 0.2 to 0.5 mm Torr. Most practical 3D and linear ion traps operate at pressures of around 1 mTorr or lower. This places a requirement for an ion trap of 100 to 150 mm length to provide a sufficiently long path length to avoid excessive ion loss. However, such long ion traps are undesirable because, for example, they result in excessively stringent manufacturing requirements. So practical ion traps have to compromise between the efficiency of ion capture and the length of the system.